1. Field of the Invention
The present invention relates generally to a receiver, and more particularly to a receiver suitable for use in mobile communication telephone systems, mobile data communication systems, and so on.
2. Description of the Related Art
Conventionally, as a receiver of the type mentioned above, a receiver employing an only-differential-detection scheme is known, for example, as disclosed in JPA-3-179852. As illustrated in FIG. 1, an only-differential-detection receiver typically includes a feedforward filter (FF filter) 102, a filter coefficient update circuit 103, a determination circuit 104, a differential detection convertor 105, and a positive/negative determination circuit 106. The feedforward filter 102 includes a plurality of cascade-connected delay elements, a plurality of multipliers respectively disposed at outputs of the plurality of delay elements, and an adder for adding output signals of the plurality of multipliers.
In the illustrated receiver, an input signal inputted from an input terminal 101 is sequentially delayed by the plurality of delay elements in the feedforward filter 102. Outputs of the respective delay elements are multiplied by the associated multipliers in the feedforward filter 102 by filter coefficients applied to the respective multipliers from the filter coefficient update circuit 103. Then, output signals of the respective multipliers are added to each other by the adder in the feedforward filter 102, and the resulting sum is inputted to the determination circuit 104 which performs demodulation. The filter coefficient update circuit 103 updates filter coefficients based on the result of the demodulation to optimize the filter coefficients. An output signal of the adder in the feedforward filter 102 is subjected to differential detection in the differential detection convertor 105, and subsequently is determined by the positive/negative determination circuit 106 whether the differential-detected output signal has a positive sign or a negative sign. The determination result of the positive/negative determination circuit 106 is outputted from an output terminal 107 as a differential detection demodulated output signal.
Since the prior art receiver determines the sign after differential detection is performed, as described above, the receiver can reduce a bit error rate (BER) due to the influence of frequency offset. However, the prior art receiver employs the only-differential-detection scheme implemented by the differential detection convertor 105. Such scheme is more susceptible to deteriorated characteristics, compared with a synchronization detection scheme, in environments free from the influence of frequency offset or fading.